1. Field of the Invention
This invention concerns a device for storing riser pipe in a semi-submersible drilling unit used in offshore oil exploration and production. More particularly, it concerns an elevator for moving the riser pipe in and out of the storage device.
2. General Discussion of the Background of the Invention
It has recently been disclosed in co-pending application Ser. No. 567,228 filed Dec. 30, 1983, now U.S. Pat. No. 4,646,672, that providing an annular, buoyant central column for semi-submersible drilling units reduces heave characteristics of semi-submersibles while providing a storage chamber for vertically oriented riser pipe. Storage of the riser pipe in a vertical orientation around the moon pool of a drilling unit, however, presented certain drawbacks.
Among the drawbacks which arose from the vertical storage of the riser pipe was the undesirability of elevating a 30-90 foot long riser pipe in a substantially vertical direction through the floor of a semi-submersible drilling unit. Vertical movement of a pipe through the floor resulted in the top of the riser pipe being presented at an inconvenient distance from the drill center of the semi-submersible unit. This was a serious drawback since the long riser pipe would then have to be moved through this distance towards the drill center line of the semi-submersible unit while the unit was being excited by waves and other environmenal forces.
The applicants of the present invention were also confronted with the problem of keeping the riser pipe under positive control along its length during storage and retrieval of the pipe to prevent it from oscillating in response to movements of the drilling unit.
Yet another problem was encountered as the center of gravity of the pipe moved through the floor and exerted harmful bending moments on running tools that were lifting the pipe.
A solution to all of these problems had to be found within the geometric limitations imposed by the central caisson. Since the cross-sectional area of the caisson which intersected the surface of the water could not exceed certain dimensions without adversely affecting the motions of the platform, a method of storing and retrieving the riser pipe had to be found that did not involve a substantial increase in the size of the annular caisson in which the pipe was stored.
Accordingly, it is an object of the invention to provide a method and apparatus for storing and retrieving riser pipe from an annular caisson around the drill center of a semi-submersible drilling unit without increasing the cross-sectional area of the caisson that intersects the water plane.
It is a further object of the invention to provide such a device that would elevate the riser pipe out of the caisson and move it through the floor of the drilling unit at an angle so that the riser pipe would be presented at a point adjacent the drill center of the seim-submersible drilling unit.
Yet another object of the invention is to provide an elevator that maintains the riser pipe under positive control along its length as the pipe is retrieved from the caisson.
Still another object of the invention is to move the riser pipe through the floor of the drilling unit at an angle such that the riser pipe can be easily controlled.
Finally it is an object of the invention to provide an elevator for riser pipe which can quickly store and retrieve the pipe.